As a process for preparing 3-norcephem compounds represented by the formula (2), there has been reported a process in which a reducing agent, such as zinc powder, is acted on a 3-halocephem compound or 3-sulfonyloxycephem compound represented by the formula (3) (JP-A-59186/1977, Recent Advances in the Chemistry of .beta.-Lactam Antibiotics, p. 170, 1977, Pure & Appl. Chem., 1987, 59, 1041, etc.) ##STR3## wherein R.sub.1 is a hydrogen atom, amino group or protected amino group, and R.sub.3 is a hydrogen atom or carboxylic acid protective group ##STR4## wherein R.sub.1 and R.sub.3 are as defined above, and X is a halogen atom, C.sub.1 .about.C.sub.3 alkanesulfonyloxy group or toluenesulfonyloxy group.
This process is, however, generally not suitable for a commercial process due to the difficulty in preparing 3-halocephem compounds or 3-sulfonyloxycephem compounds, as a starting material.
Also, there has been reported a process for preparing 3-norcephem compounds in which a 3-hydroxycephem compound is subjected to a catalytic hydrogenation to obtain a 3-hydroxycepham, followed by 1,2-elimination using haloformic acid ester/base (see JP-A-213785/1983, JP-A-34714/1983, Pure & Appl. Chem., 1987, 59, 1041, etc.) ##STR5##
This process is, however, not a commercial process because 3-hydroxycephem whose synthesis is difficult is used as a starting material and further the two steps of catalytic hydrogenation and 1,2-elimination are required.
In addition, Chemistry and Biology of .beta.-Lactam Antibiotics Penicillins and Cephalosporins Volume 1, p. 170, discloses a process in which a 3-formylcephem shown by the following reaction scheme (5) is used as a starting material, and a process employing the Wittig reaction shown by the following reaction scheme (6). ##STR6##
However, the above two processes have difficulties in obtaining their starting materials. In addition, the former requires rhodium complex that is expensive, while the latter has the problem that a large amount of phosphorus-containing waste is formed as a by-product by the Wittig reaction.
An object of the present invention is to overcome the above drawbacks of the conventional processes and provide a process which readily prepares a desired 3-norcephem compound in high yield and purity.